1. Field of the Invention
The present invention relates to a projection system in which imaging light emitted from a projector is projected on a projection screen to display thereon an image. More particularly, the present invention relates to a projection screen capable of sharply displaying an image, being improved in brightness distribution and viewing angle, providing high image visibility, and to a projection system comprising such a projection screen.
2. Background Art
A conventional projection system is usually as follows: imaging light emitted from a projector is projected on a projection screen, and viewers observe the light reflected from the projection screen as an image.
Typical examples of projection screens for use in such conventional projection systems include white-colored paper or cloth materials, and plastic films coated with inks that scatter white light. Besides, high-quality projection screens that comprise scattering layers containing beads, pearlescent pigments, or the like, capable of controlling the scattering of imaging light, are now commercially available.
Since projectors have become smaller in size and moderate in price in recent years, demand for household projectors such as projectors for family theaters is growing, and an increasing number of families are now enjoying projection systems. Household projection systems are often placed in living rooms or the like, which are usually so designed that environmental light such as sunlight and light from lighting fixtures come in abundantly. Therefore, projection screens for use in household projection systems are expected to show good image display performance even under bright environmental light.
However, the above-described conventional projection screens cannot show good image display performance under bright environmental light because they reflect not only imaging light but also environmental light such as sunlight and light from lighting fixtures.
In such a conventional projection system, differences in the intensity of light (imaging light) projected on a projection screen from a projector cause light and shade to form an image. For example, in the case where a white image on a black background is projected, the projected-light-striking part of the projection screen becomes white and the other part becomes black; thus, differences in brightness between white and black cause light and shade to form the desired image. In this case, in order to attain excellent image display, it is necessary to make the contrast between the white- and black-indication parts greater by making the white-indication part lighter and the black-indication part darker.
However, since the above-described conventional projection screen reflects both imaging light and environmental light such as sunlight and light from lighting fixtures without distinction, both the white- and black-indication parts get lighter, and differences in brightness between white and black become small. For this reason, the conventional projection screen cannot satisfactorily provide good image display unless the influence of environmental light such as sunlight and light from lighting fixtures on the projection screen is suppressed by using a means for shading a room, or by placing the projection screen in a dark environment.
Under these circumstances, studies have been made on projection screens capable of showing good image display performance even under bright environmental light. There have so far been proposed projection screens using, for example, holograms or polarized-light-separating layers (see Japanese Laid-Open Patent Publications No. 107660/1993 (Patent Document 1) and No. 540445/2002 (Patent Document 2)).
Of these conventional projection screens, those ones using holograms have the advantage that the white-indication part can be made lighter if their light-scattering effect is properly controlled, so that they can show relatively good image display performance even under bright environment light. However, holograms have wavelength selectivity but no polarization selectivity, so that the projection screens using holograms can display images only with limited sharpness. Moreover, because of production problems, it is difficult to produce large-sized projection screens by making use of holograms.
On the other hand, on the above-described conventional projection screens using polarized-light-separating layers, it is possible to make the white-indication part lighter and the black-indication part darker. Therefore, these projection screens can sharply display images even under bright environmental light as compared with the projection screens using holograms.
Specifically, Patent Document 1 describes a projection screen for which a cholesteric liquid crystal that reflects red, green and blue light (right- or left-handed circularly polarized light) contained in imaging light is used in order to make. The projection screen is constructed not to reflect nearly half the environmental light incident on the screen by making use of the circularly-polarized-light-separating property of the cholesteric liquid crystal.
However, in the projection screen described in Patent Document 1, since the cholesteric liquid crystal is in the state of planar orientation, specular reflection occurs when the cholesteric liquid crystal reflects light, and it is thus difficult to recognize the reflected light as an image. Namely, to recognize the reflected light as an image, it is necessary that the reflected light be scattered. However, Patent Document 1 is quite silent on this point.
On the other hand, Patent Document 2 describes a projection screen using, as a reflective polarization element, a multi-layered reflective polarizer or the like, having diffusing properties. This projection screen does not reflect part of the environmental light that is incident on it because of the polarized-light-separating property of the multi-layered reflective polarizer, and scatters the reflected light by interfacial reflection that occurs at an interface between materials having different refractive indices, constituting the multi-layered reflective polarizer, or by means of a diffusing element that is provided separately from the multi-layered reflective polarizer. Further, Patent Document 2 describes a projection screen using a cholesteric, reflective polarizer or the like as a reflective polarization element in combination with a diffusing element. This projection screen does not reflect part of the environmental light that is incident on it because of the polarized-light-separating property of the cholesteric, reflective polarizer, and scatters the reflected light by means of the diffusing element that is provided separately from the cholesteric, reflective polarizer.
Of the projection screens described in Patent Document 2, the former one is proposed on the premise that the reflective polarization element is a linear polarization element such as a multi-layered reflective polarizer (“DBEF” manufactured by Sumitomo 3M Limited, Japan, etc.). When this projection screen is incorporated into a projection system or the like, it is necessary to make the plane of polarization of the linear polarization element agree with the plane of polarization of a projector that emits linearly polarized light, such as a liquid crystal projector. If the planes of polarization of these two do not agree with each other, excellent image display cannot be attained.
Further, of the projection screens described in Patent Document 2, the latter one contains, as the reflective polarization element, a circular polarization element such as a cholesteric, reflective polarizer. However, since the diffusing element provided on the observation side of the reflective polarization element scatters the reflected light, the polarized-light-separating function of the reflective polarization element is impaired, and image visibility cannot be fully improved.
Namely, since the diffusing element is provided on the observation side of the reflective polarization element, light passes through the diffusing element before entering the reflective polarization element, and the state of polarization of the light is disturbed, which is called “depolarization”. Light that passes through the diffusing element includes two types of light, that is, environmental light (sunlight, etc.) and imaging light. If the state of polarization of environmental light is disturbed by the diffusing element, the light which the reflective polarization element inherently transmits is, owing to depolarization, converted into a component which the reflective polarization element reflects, and this component is reflected from the reflective polarization element as unnecessary light. On the other hand, if the state of polarization of imaging light is disturbed by the diffusing element, the light which the reflective polarization element inherently reflects is, owing to depolarization, converted into a component which the reflective polarization element does not reflect, and this component passes through the reflective polarization element. Because of these two phenomena, the original polarized-light-separating property is impaired, and image visibility cannot be fully improved.
In sum, the above-described conventional projection screens, including those ones using holograms and those ones described in Patent Documents 1 and 2, using polarized-light-separating layers, can display images only with limited sharpness under bright environmental light. Therefore, it has so far been impossible to fully improve image visibility.